Such a valve-operating system for an internal combustion engine is already known, as disclosed, for example, in Japanese Patent Application Laid-open No. 9-184409.
The known valve-operating system further includes a valve spring mounted between the valve and engine body for biasing the valve in a closing direction, and a lifter spring mounted between the subsidiary valve lifter and the valve for biasing the subsidiary valve lifter toward the subsidiary cam. When the connecting means is in the non-connecting state in which the valve lifters are free for movement, the main cam opens and closed the main valve by cooperation with the main valve lifter and the valve spring, and the subsidiary cam provides an ineffective reciprocal motion to the subsidiary valve lifter by cooperation with the lifter spring. When the connecting means is in the connecting state in which the valve lifters are connected to each other, the subsidiary cam opens and closes the valve by cooperation with both of the valve lifters and the valve spring.
It should be noted here that because the lifter spring is mounted between the valve and the subsidiary valve lifter, the following disadvantage is encountered: When the connecting means is in the non-connecting state, whereby the subsidiary cam provides the ineffective reciprocal motion to the subsidiary valve lifter by cooperation with the lifter spring, inertia forces of the subsidiary cam and the lifter spring are applied to the valve, whereby the valve is liable to be jumped. Such jumping of the valve can be suppressed by setting the preset load of the valve spring for biasing the valve in the closing direction at a higher value. However, if the preset load is set at the higher value, the following new problems arise: the load of a camshaft is increased; the wear of sliding portions is hastened, and the like.
In such known valve-operating system, the cylindrical main valve lifter and the hollow cylindrical subsidiary valve lifter are relatively slidably fitted with each other, and the outer subsidiary valve lifter is slidably received in a lifter guide bore in the engine body. Radial main and subsidiary guide bores are provided in the main and subsidiary valve lifters, and a connecting plunger is fitted in both of the guide bores, thereby connecting the valve lifters to each other. In the system designed as described above, it is necessary to provide a high-accuracy positioning means between the main and subsidiary valve lifters in order to bring the main and subsidiary guide bores in the same line with each other.
However, such positioning means results in an increase in number of parts in the valve-operating system, and moreover, causes an increase in number of processing steps, resulting in the hindrance to a reduction in cost.
Further, in such known valve-operating system, a circular upper surface of the main valve lifter and an annular upper surface of the subsidiary valve lifter are formed as slipper faces with which the main and subsidiary cams are in sliding contact. The radial main and subsidiary guide bores are provided in the main and subsidiary valve lifters, and the connecting plunger is fitted in both of the guide bores, whereby the valve lifters are connected to each other.
It should be noted here that in a valve-operating system in which a valve lifter is moved directly by a cam, it is necessary to bring the rotating cam over the entire periphery thereof reliably into sliding contact with the slipper face of the valve lifter in order to operate the valve smoothly in conformity to a cam profile. However, if even a portion of the cam strikes against a peripheral edge corner of the slipper face of the valve lifter, the following disadvantage is encountered: A large side thrust is exerted to the valve lifter, whereby the resistance to the sliding movement of the valve lifter is increased, and moreover, the movement of the valve does not correspond to the cam profile. Therefore, it is required that the slipper face of the valve lifter should have a necessary and sufficient length in a direction of rotation of the cam.
However, in the conventionally known valve-operating system in which main and subsidiary slipper faces 011s and 012s are formed at upper ends of an inner main valve lifter 011 and an outer subsidiary valve lifter 012 disposed concentrically with each other, so that main and subsidiary cams 15 and 16 are in sliding contact with the slipper faces 011s and 012s, as shown in FIGS. 52A and 52B, it is required that the slipper faces 011s and 012s have extents having band-shaped sliding contact regions A and B caused by the rotation of the main and subsidiary cams 15 and 16 in order to meet the above-described demand. This brings about an increase in diameter of the main and subsidiary valve lifters 011 and 012 and accordingly, an increase in size of the valve-operating system.